Compatibility of common MCM-D dielectrics with scanning laser ablation-via generation processes

A scanning laser ablation based via generation process for use in MCM-D (multichip module-D) substrate fabrication is described. The robustness of a variety of thin (1-5 μm-thick) conformal metal masking layers was compared by static exposure with a computer-controlled XeCl excimer laser exposure tool. The relative ablation rates of commercially available dielectric materials were compared for a range of fluences from 0 to 600 mJ/cm² using static excimer laser exposure. Based on process limits established by these static test results, generic scanning laser ablation processing parameters were established which were successfully used for the fabrication of vias in low stress, acetylene terminated and fluorinated polyimides as well as benzocyclobutene (BCB) and polyphenylquinoxalines (PPQs). The extent of soot accumulation during scanning laser ablation was dependent on the conformal metal masking layer used. In some cases, excessive accumulation of soot resulted in mask cracking around the vias. The use of helium as a process gas was found to dramatically reduce the amount of soot that accumulated and consequently eliminated soot related mask damage